Image forming apparatuses of the electrophotographic system, such as copying machines, printers, and multi-functional apparatuses (multi-functional printers) generally use a fixing apparatus employing a heat roller fixing method. The fixing apparatus includes a fixing roller and a pressure roller, which are pressed against each other. Both of or one of the fixing roller and the pressure roller are heated to a predetermined surface temperature (fixing temperature) with a heat source (for example, halogen lamp) provided inside. Printing paper with an unfixed toner image is passed through a point of contact between the fixing roller and the pressure roller (fixing nip portion), and the toner image is fixed with heat and pressure.
As the fixing roller provided in the fixing apparatus for color printers, an elastic roller with an elastic layer such as silicone rubber is commonly used. In this case, the surface of the fixing roller undergoes elastic deformation according to irregularities on the surface of the toner image, so that the surface with the toner image is covered with the fixing roller in contact with the printing paper. This is particularly suited for the fixing of unfixed toner images in color printing, which uses a larger amount of toner than monochromatic printing. Further, by the strain relieving effect of the elastic layer at the fixing nip portion, the ease of releasing toner can be improved in color printing, which readily causes offset as compared with monochromatic printing. Further, there is created a “reversed” fixing nip portion between the fixing roller and the pressure roller (the fixing roller being slightly deformed inward at the point of contact with the pressure roller). This enables the sheet to be striped more easily (self stripping), even without a stripping mechanism (stripping means) such as a stripping claw, thereby eliminating image defects caused by such stripping mechanism.
However, due to poor heat conductivity of the elastic layer, the efficiency of heat transfer suffers greatly when the heat source is provided inside the fixing roller having the elastic layer. As a result, a long warm up time is required, and, when the transport speed of sheets is increased, the surface temperature of the fixing roller cannot keep up with it.
As a countermeasure, there has been proposed a method in which the fixing roller is heated externally (from the surface) by bringing an external heating section into contact with the surface of the fixing roller (external heating and fixing method). One such method is a roller method, in which a heat roller having a heat source therein is brought into contact with the fixing roller. Another is a belt method, in which an endless belt member is heated and brought into contact with the fixing roller.
For example, Patent Publications 1 and 2 describe an external heating section employing the belt method, in which a belt member suspended by a plurality of belt-suspending rollers having heat sources therein is brought into contact with a surface of the fixing roller. As described in these publications, the belt member follows the rotation of the fixing roller by the frictional force generated on the surface of the fixing roller.
Patent Publication 1: Japanese Laid-Open Patent Publication No. 2004-198659 (published on Jul. 15, 2004)
Patent Publication 2: Japanese Laid-Open Patent Publication No. 2005-189427 (published on Jul. 14, 2005)
The belt method provides a wider heating nip area with a smaller heat capacity as compared with the roller method. This enables a large amount of heat to be supplied to the surface of the fixing roller, and therefore provides a superior temperature response in fast fixing. A problem of this method, however, is that when the fixing roller is stopped at the end of fixing, the fixing roller is locally heated in a portion where the heating member for heating the belt member is in contact with via the belt member. This advances deterioration of the fixing roller (in the following, such localized overheating will be referred to as overshoot).
That is, in the belt method, a heat supply to the fixing roller is large, and, owning to the fact that a plurality of members, such as the belt member and the belt-suspending rollers are interposed between the heat source and the surface of the fixing roller, there is a large temperature gradient between the heat source and the surface of the belt member. Thus, when the rotation of the fixing roller is stopped and the heat transfer to the fixing roller is cut off instantaneously, the temperature gradient causes temperatures of the belt-suspending rollers and belt members to rise. This causes localized overheating in the fixing roller, in a portion in contact with the belt-suspending rollers via the belt member. The problem of overshoot due to temperature gradient is particularly prominent when the belt-suspending rollers and the endless belt have small heat capacities as it is usually the case.
Further, overshoot disturbs temperature distributions on the surface of the fixing roller. This may cause uneven glossiness in images produced in the next fixing process.
Further, in the system that requires the surface temperature of the fixing roller to be varied according to printed image information, the system first turns off the heat source inside the fixing roller and the heat sources inside the belt-suspending rollers, and then rotates the fixing roller to lower the surface temperature of the fixing roller, if the surface temperature of the fixing roller needs to be decreased during a fixing operation. However, as described above, since the pre-set temperature of the belt-suspending rollers is higher than the fixing temperature, it takes time to lower the temperature of the fixing roller.
One way to overcome such a problem is to provide an external heating section that can be separated from or brought into contact with the fixing roller. Overshoot can be prevented by separating the external heating section from the fixing roller, either immediately after the rotation of the fixing roller has been stopped, or when the temperature of the fixing roller needs to be lowered. Further, with the external heating section separated from the fixing roller, the heat of the belt-suspending roller does not easily transfer to the fixing roller. This facilitates the temperature drop in the fixing roller.
However, there is a problem in the arrangement in which the external heating section is separated from the fixing roller. Specifically, the belt member deteriorates at an increased rate if the belt member is completely separated from the fixing roller. As described above, the temperature of the heating member is greater than the surface temperature of the fixing member even after the heating, and as such, if the belt member is completely separated from the fixing member, the transferred heat from the heating member concentrates on the belt member in areas around the point of contact with the heating member. This deteriorates the belt member.
Further, in the arrangement in which the belt member is completely separated from the fixing roller, the belt member needs to travel a long distance and therefore requires a large space for the release/contact. This poses space restrictions. Further, since the distance of travel is long, the release/contact mechanism requires a large and complex structure, which makes it difficult to realize the quick release/contact as attained by a simple release/contact mechanism.
Patent Publication 1 describes a release/contact structure for the external heating section, for the purpose of a jamming process and cleaning of the belt member. However, this publication does not describe a specific structure for the release/contact mechanism, nor does it mention which member of the external heating section is separated or made contact. As such, the technique described in Patent Publication 1 cannot be used to solve the problem of accelerated deterioration in the belt member or realize quick release/contact in a small space and at low power.